NORTH CHARLESTON, S.C. — Michael T. Slager played cops and robbers as a boy in the Virginia woods, volunteered as an emergency medical technician after high school and earned an associate degree in criminal justice while working full time as a patrolman.

Before he was caught on video firing eight shots at the back of an unarmed fleeing man and then dispassionately handcuffing him as he lay dying, he received praise from his supervisors at the North Charleston Police Department and excelled in police training. He was also the subject of two formal complaints in five years.

The adults who watched Mr. Slager, now 33, grow up recalled him being a shy loner who struggled to adjust to his broken home, and had a hard time socializing. They remembered him more for what he was not — not much of an athlete, not a troublemaker and not someone who spent much time with friends.

A little more than a week ago he was just a police officer in a working-class city — a homebody whose patrol car was often parked in his driveway — with a wife, expecting their first child together. Now even the police union, which is not paying for his legal defense, has distanced itself from him.

His mother, Karen Sharpe, said her son seemed unaware of just how much attention those eight shots had garnered, and how much he had come to embody law enforcement gone awry.

“I think he’s scared to death,” Ms. Sharpe said in an interview on Friday shortly after visiting her son at the Charleston County jail, where he is being held without bond. “He’s never been in this situation. Nobody ever thinks they’re going to be in this situation.”

Mr. Slager was arrested and charged with murder on Tuesday, after state investigators said he had given inconsistent accounts about the shooting of Walter L. Scott, a 50-year-old forklift operator. It is not clear why Mr. Scott ran when Mr. Slager stopped him for a broken taillight on April 4. Mr. Scott told the officer that he was in the process of buying the 1990s-era Mercedes-Benz he was driving but had not yet registered it. Mr. Scott owed $18,104 in child support payments and fees, and an arrest warrant was issued for him in 2013, according to county records.

A video shot by a barber on his way to work shows a brief tussle between the two men. Mr. Scott broke free and ran. Instead of giving chase, Mr. Slager fired his service weapon eight times as Mr. Scott staggered and collapsed to his death in a grassy field behind a pawnshop just a mile from the officer’s home.

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Mr. Slager was recorded shooting a suspect eight times.CreditCharleston County Detention Center

The video shows Mr. Slager dropping something beside Mr. Scott’s body. Most civil rights activists who watched the video believe the object was a Taser that the officer said Mr. Scott had grabbed.

The video has been viewed more than one million times on YouTube, but not by the officer’s mother.

The Police Department dismissed Mr. Slager on Wednesday, the day after his arrest.

“I can’t see him being a mean person, a bad person,” Ms. Sharpe said. “He’s a very good person,” she said. “He’s very generous. He likes to help.”

Mr. Slager was born in Florida and moved with his family to Virginia and later to southern New Jersey, where he lived with his parents and a pair of sisters in a two-story, two-car garage home on a cul-de-sac in Mount Laurel. His father, an office worker at U.P.S., used to take the family out on his sailboat, but later went through rocky patches after a failed business venture, neighbors recalled. When his parents split up in the 1990s, Mr. Slager moved with his mother and sisters back to Florida, but he soon returned to live with his father in New Jersey.

His father hired a tutor to help the adolescent with his studies. The woman, a child advocate employed at a nearby school district, wound up marrying Mr. Slager’s father and moving in, neighbors recalled. The couple also divorced, and Mr. Slager’s father now lives in Doylestown, Pa.

Helga Shay, a former neighbor in Mount Laurel, a suburban township about 15 miles outside Philadelphia, said the shooting was a stark departure from Mr. Slager’s retiring nature.

“I see him as a child of divorce,” Mrs. Shay said. “And I think that may have had an impact on him, if he was a sensitive person, and he struck me as kind of sensitive — shy and a bit quiet. He did want to talk to you and be polite. It didn’t come easy for him.”

She remembered him washing the family’s cars, doing chores like yard work and being helpful to his parents.

“Just a nice kid, you know,” said Nancy Thomas, another former neighbor. “He was a little shy,” she added.

After graduating from Lenape Regional High School in 2001 at the age of 19, Mr. Slager volunteered at Mount Laurel Emergency Medical Services, where he took dozens of hours of emergency medical training, including CPR.

“I remember him being always very respectful to me — you know, he said, ‘Yes, sir, no, sir,’ and he did what was expected of him,” his former chief, Fran Pagurek, said by phone. “We never had an issue with Mike while he was here.”

He worked briefly as a waiter in New Jersey, and then in 2003, Mr. Slager joined the Coast Guard. He served for six years, mostly in Florida, before joining the North Charleston Police Department.

In 2010, the year he joined the Police Department, he married a woman with two children. They lived in Goose Creek, a suburb, where the children, a boy and a girl, played on a trampoline in the backyard when they were not being home-schooled by their mother, Jamie.

Mr. Slager told neighbors that he was waiting to be called for a government job out West. When he sold his house and moved last year, they assumed that was where he had gone. He had actually moved only 10 miles away, into a house on a busy street much closer to his job.

“Officer Slager on his first day of training was very enthused and ready to work,” an officer, Dan Bailey, wrote in a March 2010 report. “He wanted to be involved.”

He earned early praise for his handling of an encounter with an armed person and, in one instance, demonstrating “a great relationship with a citizen who was providing officers with information that a resident in a community had been shot at.”

Video

In video provided to The New York Times, a police officer in North Charleston, S.C., is seen shooting an apparently unarmed man after a scuffle following a traffic stop.

In 2013, a North Charleston man filed a complaint against Mr. Slager, accusing the officer of arriving at his house in pursuit of a burglary suspect and using a Taser on him on the steps of his own home.

City records show that the man, Mario Givens, told authorities that he had explained to Mr. Slager that the suspect he sought was 5-foot-5. Mr. Givens was 6-foot-3. The burglary victim, who was waiting outside in her car, shouted to the officer that Mr. Givens was not the burglar, according to the records.

Mr. Slager was exonerated, according to city documents.

“I was upset, because technically, they took a real long time to investigate,” Mr. Givens said at a news conference on Thursday, where he announced intentions to sue. “They kept telling me: ‘We’re still investigating. We’re still investigating.’ ”

Had the city followed up, he said, maybe Mr. Scott would still be alive, because Mr. Slager would have been removed from the force.

Mr. Givens declined to comment further on Thursday, and no one answered the door at his home on Friday.

Also in 2013, Mr. Slager received an associate degree in criminal justice from Trident Technical College, where he also studied civil engineering.

His wife is expecting their first child together in May.

“He went to all the baby appointments, all the ultrasound appointments,” his mother, Ms. Sharpe, said. “It saddens me to know that he won’t be in the delivery room.”

Mrs. Shay, the former neighbor, spoke on Saturday with the officer’s stepmother, who was crying and puzzled over the latest turn of events and wondering whether Mr. Slager had become “desensitized” after joining the Police Department.

“She and I are agreeing on the fact that we both believe that something happened in the training,” Mrs. Shay said. “She agrees with me. I, from the beginning, thought this is not Michael at all.”

Mr. Slager’s lawyer, Andrew J. Savage III, has said little about the case. “We are continuing our effort to methodically investigate all the facts and circumstances which led to Officer Slager’s arrest,” said Mr. Savage, who took the case after the first lawyer quit after the shooting video surfaced. “We have a long way to go.”